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Shock absorber A study of the leg muscles of leaping toads might explain why we experience a jolting impact when we step off an unexpected stair.

Dr Emanuel Azizi and colleague Emily Abbott from the University of California, Irvine used implanted sensors to study how toads protect their leg muscles from overstretching when they land. They found the anconeus muscle - located in the forearm near the elbow - contracts just before they land, in proportion to the distance of the leap.

"Our results show that shortening of the anconeus muscle prior to landing is tuned to hop distance and, therefore, functions to protect the muscle from being actively stretched to long lengths during the landing phase," the researchers write.

"Any time you decelerate your body, your muscles act as the brakes and they have to be stretched in order to do that," says Azizi, who is an assistant professor of biology.

However there is a risk that the muscles will overstretch upon landing, causing damage to the muscle. Shortening the muscle just before landing can minimise this risk, Azizi says.

"You're stretching the same distance but you're starting from a shorter length."

The results suggest that toads use a variety of cues to decide how much to shorten the muscle, based on the anticipated degree of impact.

"We think it could be that they sense how much muscle power they're using when they jump so they could process that information and say, 'If I'm jumping this far, or if I'm using my jumping muscles to a certain degree, that's going to tell me some information about what the impact's going to be when I land."

Unanticipated jolt

This could explain why, in the absence of those cues or when those cues are misleading - such as an unanticipated step - our muscles are not prepared for the impact and we experience a jolt or even muscle damage.

"When you're walking normally or jumping off a relatively short platform, you're acquiring a lot of sensory information that's helping you tune your muscles to that impact event," says Azizi.

"When you're fooled in terms of your sensory information and your muscles aren't prepared for it, you feel that jolt and you feel that something didn't go right."